Graphene, power storage device, and electric device

ABSTRACT

An object is to provide graphene which has high conductivity and is permeable to ions of lithium or the like. Another object is to provide, with use of the graphene, a power storage device with excellent charging and discharging characteristics. Graphene having a hole inside a ring-like structure formed by carbon and nitrogen has conductivity and is permeable to ions of lithium or the like. The nitrogen concentration in graphene is preferably higher than or equal to 0.4 at. % and lower than or equal to 40 at. %. With use of such graphene, ions of lithium or the like can be preferably made to pass; thus, a power storage device with excellent charging and discharging characteristics can be provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to graphene or a stack of graphene whichhas high conductivity and is permeable to ions of lithium or the like.Such graphene or stack of graphene is used for a material for alithium-ion secondary battery, or the like.

Note that in this specification and the like, graphene refers to aone-atom-thick sheet of carbon molecules having sp² bonds, and a stackof graphene refers to a structure in which a plurality of one-atom-thicksheets (the number of sheets is greater than or equal to 2 or less thanor equal to 100, preferably greater than or equal to 2 or less than orequal to 50) of carbon molecules having sp² bonds are stacked.

2. Description of the Related Art

Owing to excellent electric characteristics such as high conductivity orhigh mobility and excellent physical characteristics such as sufficientflexibility and high mechanical strength, application of graphene to avariety of products has been attempted (see Patent Documents 1 to 3). Inaddition, a technique for applying graphene to a lithium-ion secondarybattery has been proposed (see Patent Document 4).

REFERENCE [Patent Document 1] United States Patent ApplicationPublication No. 2011/0070146 [Patent Document 2] United States PatentApplication Publication No. 2009/0110627 [Patent Document 3] UnitedStates Patent Application Publication No. 2007/0131915 [Patent Document4] United States Patent Application Publication No. 2010/0081057 SUMMARYOF THE INVENTION

Graphene has high conductivity, but permeability of graphene to ions hasnot yet been elucidated sufficiently. An object of one embodiment of thepresent invention is to provide graphene which has high conductivity andpermeability to ions of lithium or the like. Another object is toprovide a power storage device with excellent charge and dischargecharacteristics by using the graphene. Further another object is toprovide an electric device which has high reliability and can withstandlong-term or repeated use. The present invention achieves at least oneof the above objects.

A power storage device according to one embodiment of the presentinvention includes a positive electrode including a positive electrodeactive material layer provided over a positive electrode currentcollector; a negative electrode including a negative electrode activematerial layer which is provided over a negative electrode currentcollector and includes a negative electrode active material and graphenehaving a hole; and a separator provided between the positive electrodeand the negative electrode; and an electrolyte. A hole is formed in thegraphene, whereby a path through which an ion passes can be formed. Notethat in this specification and the like, the term “hole” indicates theinside of a ring-like structure formed by carbon and nitrogen, or formedby carbon and one or more elements selected from a Group 16 element suchas oxygen or sulfur, and halogen such as chlorine. Note that thegraphene may have one or more holes.

A power storage device according to one embodiment of the presentinvention includes a positive electrode including a positive electrodeactive material layer provided over a positive electrode currentcollector; a negative electrode including a negative electrode activematerial layer which is provided over a negative electrode currentcollector and includes a negative electrode active material and graphenehaving a hole; a separator provided between the positive electrode andthe negative electrode; and an electrolyte. A hole is formed ingraphene, whereby a path through which an ion passes can be formed.

A power storage device according to another embodiment of the presentinvention includes a positive electrode including a positive electrodeactive material layer which is provided over a positive electrodecurrent collector and includes a positive electrode active material andgraphene having a hole; a negative electrode including a negativeelectrode active material layer provided over a negative electrodecurrent collector; a separator provided between the positive electrodeand the negative electrode; and an electrolyte.

A power storage device according to another embodiment of the presentinvention includes a positive electrode including a positive electrodeactive material layer which is provided over a positive electrodecurrent collector and includes a positive electrode active material andgraphene having a hole; a negative electrode including a negativeelectrode active material layer which is provided over a negativeelectrode current collector and includes a negative electrode activematerial and graphene having a hole; a separator provided between thepositive electrode and the negative electrode; and an electrolyte.

In the above structures, the nitrogen concentration in graphene ispreferably higher than or equal to 0.4 at. % and lower than or equal to40 at. %.

The graphene having a hole may have a stacked structure.

Another embodiment of the present invention is an electric deviceincluding the power storage device having the above structure.

In a power storage device according to one embodiment of the presentinvention, graphene which has a hole or a stack of such graphene is usedfor at least one of a positive electrode active material layer and anegative electrode active material layer. The hole formed in thegraphene serves as a path through which ions pass; thus, the ion iseasily inserted or extracted into/from the active material. Accordingly,charging and discharging characteristics of the power storage device canbe improved.

Further, when graphene having a hole or a stack of such graphene is usedfor at least one of the positive electrode active material layer and thenegative electrode active material layer, decomposition of theelectrolyte on a surface of the active material layer does not easilyoccur, and thus the thickness of a surface film which is deposited onthe surface of the active material layer and inhibits insertion andextraction of ions can be small. With such a structure, ions are easilyinserted or extracted into/from the active material; accordingly,charging and discharging characteristics of the power storage device canbe improved.

According to one embodiment of the present invention, graphene which hashigh conductivity and is permeable to ions of lithium or the like can beprovided. Further, with use of the graphene, a power storage devicewhich has excellent charging and discharging characteristics can beprovided. Furthermore, with use of the power storage device, an electricdevice which has high reliability and can withstand long-term orrepeated use can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are pattern diagrams of graphene.

FIG. 2 is a graph showing potential of vacancies.

FIG. 3 illustrates a structure of a coin-type secondary battery.

FIG. 4 illustrates electric devices.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below. Note that embodiments can becarried out in many different modes, and it is easily understood bythose skilled in the art that modes and details of the present inventioncan be modified in various ways without departing from the spirit andthe scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments.

Embodiment 1

In this embodiment, an example in which graphene having a path throughwhich an ion passes or a stack of graphene having a path through whichan ion passes is formed on a surface of silicon particles, will bedescribed.

First, graphite oxide is prepared by oxidizing graphite and thensubjected to ultrasonic vibration to give graphene oxide. For details,Patent Document 2 may be referred to. Alternatively, commerciallyavailable graphene oxide may be used.

Next, the graphene oxide is mixed with silicon particles. The proportionof graphene oxide may be set in the range from 1 wt. % to 15 wt. %,preferably from 1 wt. % to 5 wt. % of the total.

Then, heat treatment is performed on the mixture of the graphene oxideor a stack of graphene oxide and the silicon particles in an ammoniaatmosphere at 150° C. or higher, preferably 200° C. or higher. Note thatgraphene oxide is known to be reduced at 150° C.

By the heat treatment, the graphene oxide formed over the surfaces ofthe silicon particles is reduced to be graphene. In this treatment,graphene and another adjacent graphene are bonded to form a largernet-like or sheet-like network, and at this time, a hole is formed inthe graphene. When the heat treatment is performed in an ammoniaatmosphere, nitrogen is added to the graphene. The nitrogenconcentration in the graphene is preferably higher than or equal to 0.4at. % and lower than or equal to 40 at. %. Further, when a plurality ofgraphene sheets (the number of sheets is greater than or equal to 2 orless than or equal to 100, preferably greater than or equal to 2 or lessthan or equal to 50) are formed, a stack of graphene is formed.

Alternatively, heat treatment may be performed on the mixture of thegraphene oxide and silicon particles in vacuum or an inert gas (such asnitrogen or a rare gas) atmosphere at 150° C. or higher, preferably 200°C. or higher, and then nitrogen may be added to graphene or a stack ofgraphene. The hole can be formed in the graphene by the above method.

Alternatively, by performing ammonia plasma treatment or nitrogen plasmatreatment, nitrogen can be added to graphene or a stack of graphene.Further alternatively, a nitrogen ion may be added to graphene or astack of graphene by an ion doping method or an ion implantation method.

An atom substituted for a carbon atom by implantation of element may bean oxygen atom, a halogen atom such as a chlorine atom, or a sulfuratom, instead of a nitrogen atom. In this case, the hole indicates theinside of a ring-like structure formed by carbon and one or moreelements selected from a Group 16 element such as oxygen or sulfur, andhalogen such as chlorine.

Here, a graphene structure having a hole where a lithium ion passes,will be described with reference to FIGS. 1A and 1B and FIG. 2. FIGS. 1Aand 1B schematically illustrate lattice structures of graphene. FIG. 1Ashows a case where a lithium ion approaches a vacancy caused byextraction of one carbon atom from graphene. FIG. 1B shows a case wherea lithium ion approaches vacancies caused as follows: two carbon atomsadjacent to each other are extracted from graphene, other two carbonatoms which are nearest neighbors to one of the above two carbon atomsare extracted and substituted with nitrogen atoms, and still other twocarbon atoms which are neighbors to the other one of the above twocarbon atoms are extracted and substituted with nitrogen atoms.

FIG. 2 shows results of calculation of potential energy between alithium ion and a vacancy in graphene in FIG. 1A and FIG. 1B, byfirst-principles calculation. In FIG. 2, the horizontal axis indicates adistance (nm) from graphene, and the vertical axis indicates energy(eV). In addition, in FIG. 2, a curve A represents potential of thevacancy in graphene of FIG. 1A, and a curve B represents potential ofthe vacancy in graphene of FIG. 1B.

In FIG. 2, the potential between the lithium ion and the vacancy ingraphene, represented by the curve A, has a minimal energy when thedistance between the vacancy in graphene and the lithium ion is around0.2 nm. When the distance is further decreased, the energy reverselyincreases. When the distance between the vacancy in graphene and thelithium ion is 0 nm, the energy is 3 eV. Thus, the lithium ion needs 3eV of energy to reach the vacancy in graphene. From this result, it isfound that the structure of FIG. 1A has difficulty in making the lithiumion pass through the vacancy in graphene.

On the other hand, the potential energy between the lithium ion and thevacancy in graphene, represented by the curve B, gradually decreases asthe distance between the vacancy in graphene and the lithium ion becomesdecreased, which means generation of attraction force between thelithium ion and the vacancy (hole) in the graphene and disappearance ofenergy barrier. From this result, it is found that the structure of FIG.1B facilitates passing of the lithium ion through the hole.

Since graphene is a one-atom-thick sheet of carbon molecules having sp²bonds, it is difficult for an ion of lithium or the like to pass throughthe graphene. However, graphene according to one embodiment of thepresent invention has a hole (path through which an ion passes); thus,an ion of lithium or the like can easily pass through the graphene.Further, even when a plurality of sheets of graphene are stacked, an ioncan easily pass because the graphene has a path for an ion passing.Further, as a preferable structure, a ring-like structure which has thehole therein is formed by carbon and nitrogen, so that an ion can easilypass through the structure.

As an ion which passes through graphene, an alkali metal ion such as alithium ion, a sodium ion, or potassium ion, an alkali earth metal ionsuch as a calcium ion, a strontium ion, or a barium ion, a berylliumion, a magnesium ion, or the like can be given. A hole in the graphenepreferably has such a size that the aforementioned ion can pass throughthe hole. Note that the hole in the graphene may form a many-memberedring which is a nine- or more-membered ring. Also, the graphene have mayone or more holes.

The silicon particles having been subjected to the above treatments aredispersed in an appropriate solvent (preferably a polar solvent such aswater, chloroform, N,N-dimethylformamide (DMF), or N-methylpyrrolidone(NMP)) to obtain a slurry. A secondary battery can be manufactured usingthe slurry.

FIG. 3 schematically illustrates a structure of a coin-type secondarybattery. As illustrated in FIG. 3, the coin-type secondary batteryincludes a negative electrode 204, a positive electrode 232, a separator210, an electrolyte (not illustrated), a housing 206, and a housing 244.Besides, the coin-type secondary battery includes a ring-shapedinsulator 220, a spacer 240, and a washer 242. The negative electrode204 and the positive electrode 232 are provided so as to face eachother, and the separator 210 is provided therebetween.

The negative electrode 204 includes a negative electrode active materiallayer 202 over a negative electrode current collector 200.

For the negative electrode current collector 200, a conductive materialcan be used, for example. Examples of the conductive material includealuminum (Al), copper (Cu), nickel (Ni), and titanium (Ti). In addition,an alloy material including two or more of the above-mentionedconductive materials can be used as the negative electrode currentcollector 200. Examples of the alloy material include an Al—Ni alloy andan Al—Cu alloy. The negative electrode current collector 200 can have afoil shape, a plate shape, a net shape, or the like as appropriate.Further, the negative electrode current collector 200 can be formed insuch a manner that a conductive layer is formed over another formationsubstrate, and the conductive layer is separated from the formationsubstrate.

There is no particular limitation on the material for the negativeelectrode active material layer 202 as long as it is a material withwhich metal can be dissolved/precipitated or a material into/from whichmetal ions can be inserted/extracted. For the negative electrode activematerial, a lithium metal, a carbon-based material, silicon, a siliconalloy, or tin can be used, for example. As the carbon-based materialinto/from which a lithium ion can be inserted/extracted, a fine graphitepowder, a graphite fiber, or graphite can be used. The negativeelectrode active material layer 202 may be formed in such a manner thatthe above-described slurry alone or in combination with a binder isapplied onto the negative electrode current collector 200 and dried.

The positive electrode 232 includes a positive electrode active materiallayer 230 on a positive electrode current collector 228. Note that inFIG. 3, the positive electrode 232 is placed upside down.

As the positive electrode current collector 228, a conductive materialsimilar to that of the negative electrode current collector 200 can beused.

For the positive electrode active material, a material including ions toserve as carriers and a transition metal can be used, for example. Asthe material including ions to serve as carriers and a transition metal,a material represented by a general formula A_(h)M_(i)PO_(j) (h>0, i>0,j>0) can be used, for example. Here, A represents, for example, analkaline metal such as lithium, sodium, or potassium; or an alkalineearth metal such as calcium, strontium, or barium; beryllium; ormagnesium. M represents a transition metal such as iron, nickel,manganese, or cobalt, for example. Examples of the material representedby the general formula A_(h)M_(i)PO_(j) (h>0, i>0, j>0) include lithiumiron phosphate and sodium iron phosphate. The material represented by Aand the material represented by M may be selected from one or more ofeach of the above materials.

Alternatively, a material represented by a general formulaA_(h)M_(i)O_(j) (h>0, i>0, j>0) can be used. Here, A represents, forexample, an alkaline metal such as lithium, sodium, or potassium; or analkaline earth metal such as calcium, strontium, or barium; beryllium;or magnesium. M indicates a transition metal such as iron, nickel,manganese, or cobalt. Examples of the material represented by thegeneral formula A_(h)M_(i)O_(j) (h>0, i>0, j>0) include lithiumcobaltate, lithium manganate, and lithium nickelate. The materialrepresented by A and the material represented by M may be selected fromone or more of each of the above materials.

In the case of a lithium-ion secondary battery, a material includinglithium is preferably selected for the positive electrode activematerial. In other words, A in the above general formulaeA_(h)M_(i)PO_(j) (h>0, i>0, j>0) or A_(h)M_(i)O_(j) (h>0, i>0, j>0) ispreferably lithium.

A positive electrode active material layer 230 may be formed in suchmanner that slurry in which the positive electrode active materialparticles are mixed together with a binder and a conduction auxiliaryagent, is applied onto the positive electrode current collector 228 andthen dried.

The size of the active material particles is preferably 20 nm to 100 nm.Further, a carbohydrate such as glucose may be mixed at the time ofbaking of the positive electrode active material particles so that thepositive electrode active material particles are coated with carbon.This treatment can improve the conductivity.

Further, as slurry used for forming the positive electrode activematerial layer 230, the graphene oxide may be mixed in addition to thepositive electrode active material. Slurry including at least thepositive electrode active material and the graphene oxide is appliedonto the positive electrode current collector 228, dried, and reduced,whereby the positive electrode active material layer 230 including thepositive electrode active material and graphene can be formed.

For the electrolyte, electrolyte salt dissolved in the nonaqueoussolvent may be used. As the nonaqueous solvent, a mixed solvent ofethylene carbonate (EC) and diethyl carbonate (DEC), an ionic liquidincluding quaternary ammonium-based cations, or an ionic liquidincluding imidazolium-based cations can be used. The electrolyte saltmay be electrolyte salt which includes ions serving as carriers andcorresponds with the positive electrode active material layer 230. Forexample, lithium chloride (LiCl), lithium fluoride (LiF), lithiumperchlorate (LiClO₄), lithium fluoroborate (LiBF₄), LiAsF₆, LiPF₆,Li(CF₃SO₂)₂N, and the like are preferably used, but the electrolyte saltis not limited thereto.

An insulator with pores (e.g., polypropylene) may be used for theseparator 210. Alternatively, a solid electrolyte which is permeable tolithium ions may be used.

The housing 206, the housing 244, the spacer 240, and the washer 242each of which is made of metal (e.g., stainless steel) are preferablyused. The housing 206 and the housing 244 have a function ofelectrically connecting the negative electrode 204 and the positiveelectrode 232 to the outside.

The negative electrode 204, the positive electrode 232, and theseparator 210 are soaked in the electrolyte. Then, as illustrated inFIG. 3, the negative electrode 204, the separator 210, the ring-shapedinsulator 220, the positive electrode 232, the spacer 240, the washer242, and the housing 244 are stacked in this order with the housing 206positioned at the bottom. The housing 206 and the housing 244 aresubjected to pressure bonding. In such a manner, the coin-type secondarybattery is manufactured.

Note that a method for manufacturing the lithium ion-secondary batteryis described in FIG. 3. However, the power storage device of oneembodiment of the present invention is not limited thereto. A capacitoris used as the power storage device according to one embodiment of thepresent invention. More specifically, a lithium ion capacitor or anelectric double layer capacitor is used as the capacitor.

In the power storage device according to one embodiment of the presentinvention, graphene which has a hole and or a stack of such graphene isused for at least one of a positive electrode active material layer anda negative electrode active material layer. The hole formed in thegraphene serves as a path through which an ion passes; thus, insertionor extraction of ions into/from an active material becomes easy.Accordingly, charging and discharging characteristics of the powerstorage display can be improved.

Further, when graphene having a hole or a stack of such graphene is usedfor at least one of the positive electrode active material layer and thenegative electrode active material layer, decomposition of theelectrolyte on a surface of the active material layer does not easilyoccur, and thus a surface film which is deposited on the surface of theactive material layer and inhibits insertion and extraction of ions canbe made thin. With such a structure, ions are easily inserted orextracted into/from the active material; accordingly, charging anddischarging characteristics of the power storage device can be improved.

This embodiment can be implemented by being combined as appropriate withany of the other embodiments.

Embodiment 2

In this embodiment, an example of forming graphene having a path throughwhich an ion passes or a stack of such graphene on a surface of anactive material layer including silicon formed over a current collector,will be described.

First, graphene oxide is dispersed in a solvent such as water or NMP.The solvent is preferably a polar solvent. The concentration of grapheneoxide may be 0.1 g to 10 g per liter.

A current collector provided with an active material layer includingsilicon is immersed in this solution, taken out, and then dried.

Further, a mixture of the graphene oxide and silicon particles is heatedin an ammonia atmosphere. The temperature of the heat treatment may behigher than or equal to 150° C. and lower than or equal to a meltingpoint of a conductive material used for the current collector. Throughthe above steps, graphene which has a hole or a stack of such graphenecan be formed on a surface of the silicon active material layer.

Note that after the graphene or the stack of such graphene is formed inthe above manner, the same process may be repeated, so that a stack ofgraphene can be additionally formed. The process may be repeated threeor more times. It is preferable that a stack of graphene be formed insuch a manner because the strength of graphene increases.

In the case where a thick stack of graphene is formed at a time, thedirection of the sp² bonds in the graphene is disordered, and thestrength of the stack of graphene is not proportional to the thickness.On the other hand, in the case where a stack of graphene is formedthrough a plurality of steps as described above, the sp² bonds in thegraphene are substantially parallel to a silicon surface and thereforethe strength of the stack of graphene increases as the thicknessincreases.

Since in the stack of graphene, each graphene has a hole, the stackstructure does not inhibit transfer of lithium. Thus, an ion can beeasily inserted or extracted into/from the active material. Accordingly,charging and discharging characteristics of the power storage device canbe improved.

This embodiment can be implemented by being combined as appropriate withany of the other embodiments.

Embodiment 3

In this embodiment, another example of forming graphene having a paththrough which an ion passes or a stack of such graphene on a surface ofa silicon active material layer formed over a current collector, will bedescribed.

As in Embodiment 2, graphene oxide is dispersed in a solvent such aswater or NMP. The concentration of graphene oxide may be 0.1 g to 10 gper liter.

A current collector provided with a silicon active material layer is putin the solution in which the graphene oxide is dispersed, and this isused as a positive electrode. A conductor serving as a cathode is put inthe solution, and an appropriate voltage (e.g., 5 V to 20 V) is appliedbetween the positive electrode and the negative electrode. In thegraphene oxide, part of an edge of a graphene sheet with a certain sizeis terminated by a carboxyl group (—COOH), and therefore, in a solutionsuch as water, hydrogen ions are released from the carboxyl group andthe graphene oxide itself is negatively charged and thus attracted toand deposited onto the positive electrode. Note that the voltage at thistime is not necessarily constant. By measurement of the amount ofelectric charge flowing between the positive electrode and the negativeelectrode, the thickness of a layer of graphene oxide deposited on thesilicon active material layer can be estimated.

When a graphene oxide with a necessary thickness is obtained, thecurrent collector is taken out of the solution and dried.

Further, a mixture of the graphene oxide and silicon particles is heatedin an ammonia atmosphere. The temperature of the heat treatment may behigher than or equal to 150° C. and lower than equal to a melting pointof a conductive material used for the current collector. Through theabove steps, graphene which has a hole or a stack of such graphene canbe formed on a surface of the silicon active material layer.

Even when the silicon active material has projections and depressions,the graphene formed as described above has a substantially uniformthickness even at the projections and depressions. In the above manner,graphene having a path through which an ion passes or a stack of suchgraphene can be formed on a surface of the silicon active materiallayer. The thus formed graphene or stack of graphene has a hole asdescribed above and accordingly are permeable to ions accordingly.

Note that after the graphene or the stack of graphene is formed in theabove manner, formation of graphene or a stack of graphene with themethod described in this embodiment or Embodiment 2 may be performedonce or plural times.

This embodiment can be implemented by being combined as appropriate withany of the other embodiments.

Embodiment 4

The power storage device according to one embodiment of the presentinvention can be used as a power supply of various electric deviceswhich are driven by electric power.

The following are given as specific examples of electric devices usingthe power storage device according to one embodiment of the presentinvention, display devices, lighting devices, desktop personal computersor laptop personal computers, image reproduction devices which reproducea still image or a moving image stored in a recording medium such as adigital versatile disc (DVD), mobile phones, portable game machines,portable information terminals, e-book readers, cameras such as videocameras or digital still cameras, high-frequency heating apparatusessuch as microwaves, electric rice cookers, electric washing machines,air-conditioning systems such as air conditioners, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, dialysis devices, and the like. Inaddition, moving objects driven by an electric motor using electricpower from a power storage device are also included in the category ofelectric devices. As examples of the moving objects, electric vehicles,hybrid vehicles which include both an internal-combustion engine and amotor, motorized bicycles including motor-assisted bicycles, and thelike can be given.

In the electric devices, the power storage device according to oneembodiment of the present invention can be used as a power storagedevice for supplying enough electric power for almost the whole powerconsumption (referred to as a main power supply). Alternatively, in theelectric devices, the power storage device according to one embodimentof the present invention can be used as a power storage device which cansupply electric power to the electric devices when the supply of powerfrom the main power supply or a commercial power supply is stopped (sucha power storage device is referred to as an uninterruptible powersupply). Further alternatively, in the electric devices, the powerstorage device according to one embodiment of the present invention canbe used as a power storage device for supplying electric power to theelectric devices, which is used together with the main power supply or acommercial power supply (such a power storage device is referred to asan auxiliary power supply).

FIG. 4 illustrates specific structures of the electric devices. In FIG.4, a display device 5000 is an example of an electric device including apower storage device 5004 according to one embodiment of the presentinvention. Specifically, the display device 5000 corresponds to adisplay device for TV broadcast reception and includes a housing 5001, adisplay portion 5002, speaker portions 5003, the power storage device5004, and the like. The power storage device 5004 according to oneembodiment of the present invention is provided inside the housing 5001.The display device 5000 can receive electric power from a commercialpower supply. Alternatively, the display device 5000 can use electricpower stored in the power storage device 5004. Thus, the display device5000 can be operated with use of the power storage device 5004 accordingto one embodiment of the present invention as an uninterruptible powersupply even when electric power cannot be supplied from the commercialpower supply because of power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), a field emission display (FED), and the like can be used for thedisplay portion 5002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 4, an installation lighting device 5100 is an example of anelectric device including a power storage device 5103 according to oneembodiment of the present invention. Specifically, the lighting device5100 includes a housing 5101, a light source 5102, the power storagedevice 5103, and the like. FIG. 4 shows the case where the power storagedevice 5103 is provided in a ceiling 5104 on which the housing 5101 andthe light source 5102 are installed; alternatively, the power storagedevice 5103 may be provided in the housing 5101. The lighting device5100 can receive electric power from the commercial power supply.Alternatively, the lighting device 5100 can use electric power stored inthe power storage device 5103. Thus, the lighting device 5100 can beoperated with use of the power storage device 5103 according to oneembodiment of the invention as an uninterruptible power supply even whenpower cannot be supplied from the commercial power supply due to powerfailure or the like.

Note that although the installation lighting device 5100 provided in theceiling 5104 is illustrated in FIG. 4 as an example, the power storagedevice according to one embodiment of the present invention can be usedin an installation lighting device provided in, for example, a wall5105, a floor 5106, a window 5107, or the like other than the ceiling5104. Alternatively, the power storage device can be used in a tabletoplighting device and the like.

As the light source 5102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, adischarge lamp such as an incandescent lamp and a fluorescent lamp, anda light-emitting element such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 4, an air conditioner including an indoor unit 5200 and anoutdoor unit 5204 is an example of an electric device including a powerstorage device 5203 according to one embodiment of the presentinvention. Specifically, the indoor unit 5200 includes a housing 5201, aventilation duct 5202, the power storage device 5203, and the like. FIG.4 shows the case where the power storage device 5203 is provided in theindoor unit 5200; alternatively, the power storage device 5203 may beprovided in the outdoor unit 5204. Further alternatively, the powerstorage devices 5203 may be provided in both the indoor unit 5200 andthe outdoor unit 5204. The air conditioner can receive power from thecommercial power supply. Alternatively, the air conditioner can usepower stored in the power storage device 5203. Specifically, in the casewhere the power storage devices 5203 are provided in both the indoorunit 5200 and the outdoor unit 5204, the air conditioner can be operatedwith use of the power storage device 5203 according to one embodiment ofthe present invention as an uninterruptible power supply even whenelectric power cannot be supplied from the commercial power supplybecause of power failure or the like.

Note that although the separated air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 4 as an example, thepower storage device according to one embodiment of the presentinvention can be used in an air conditioner in which the functions of anindoor unit and an outdoor unit are integrated in one housing.

In FIG. 4, an electric refrigerator-freezer 5300 is an example of anelectric device including a power storage device 5304 according to oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 5300 includes a housing 5301, a refrigerator door5302, a freezer door 5303, the power storage device 5304, and the like.The power storage device 5304 is provided in the housing 5301 in FIG. 4.The electric refrigerator-freezer 5300 can receive electric power fromthe commercial power supply. Alternatively, the electricrefrigerator-freezer 5300 can use electric power stored in the powerstorage device 5304. Thus, the electric refrigerator-freezer 5300 can beoperated with use of the power storage device 5304 according to oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from the commercial powersupply because of power failure or the like.

Note that among the electric devices described above, a high-frequencyheating apparatus such as a microwave and an electric device such as anelectric rice cooker require high power in a short time. The tripping ofa circuit breaker of a commercial power supply in use of electricdevices can be prevented by using the power storage device according toone embodiment of the present invention as an auxiliary power supply forsupplying electric power which cannot be supplied enough by a commercialpower supply.

In addition, in a time period when electric devices are not used,specifically when a rate of actual use of electric power with respect tothe total amount of electric power which can be supplied by a commercialpower supply source (such a rate referred to as a usage rate of electricpower) is low, electric power can be stored in the power storage device,whereby the usage rate of electric power can be reduced in a time periodwhen the electric devices are used. In the case of the electricrefrigerator-freezer 5300, electric power can be stored in the powerstorage device 5304 at night time when the temperature is low and therefrigerator door 5303 and the freezer door 5302 are not opened orclosed. The power storage device 5304 is used as an auxiliary powersupply in daytime when the temperature is high and the refrigerator door5303 and the freezer door 5302 are opened and closed; thus, the usagerate of electric power in daytime can be reduced.

In the power storage device according to one embodiment of the presentinvention, graphene which has high conductivity and is permeable to ionsof lithium or the like is used. With use of the graphene in the powerstorage device, the power storage device can have excellent charge anddischarge characteristics. Further, an electric device provided with thepower storage device has high reliability and can withstand long-term orrepeated use.

This embodiment can be implemented by being combined as appropriate withany of the other embodiments.

This application is based on Japanese Patent Application serial no.2011-140743 filed with Japan Patent Office on Jun. 24, 2011, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A method of manufacturing an electrode of a powerstorage device comprising: forming a graphene oxide on a particle; andreducing the graphene oxide by heat treatment in an atmospherecontaining nitrogen so that a graphene doped with nitrogen is formed onthe particle, wherein the graphene comprises at least one hole inside aring-like structure formed by carbon and nitrogen.
 3. The methodaccording to claim 2, wherein a nitrogen concentration in the grapheneis higher than or equal to 0.4 at. % and lower than or equal to 40 at.%.
 4. The method according to claim 2, wherein the hole is a nine- ormore-membered ring.
 5. The method according to claim 2, wherein theatmosphere contains ammonia.
 6. The method according to claim 2, whereinthe heat treatment is performed at 150° C. or higher.
 7. The methodaccording to claim 2, wherein the particle is made of silicon.
 8. Amethod of manufacturing an electrode of a power storage devicecomprising: forming a mixture containing a graphene oxide and aparticle; heating the mixture to reduce the graphene oxide and form agraphene on the particle; and adding nitrogen to the graphene afterperforming the heating, wherein the graphene comprises at least one holeinside a ring-like structure formed by carbon and nitrogen.
 9. Themethod according to claim 8, wherein the step of heating is performed ina vacuum or in an inert gas.
 10. The method according to claim 8,wherein a nitrogen concentration in the graphene is higher than or equalto 0.4 at. % and lower than or equal to 40 at. %.
 11. The methodaccording to claim 8, wherein the hole is a nine- or more-membered ring.12. The method according to claim 8, wherein the step of heating isperformed at 150° C. or higher.
 13. The method according to claim 8,wherein the particle is made of silicon.